1. Field of the Invention
The present invention relates to a bulk degaussing system for erasing of various magnetic media generally used for the storage of information; the system including a pair of coils wound with intersecting axes, for example, orthogonal to each other and in close conformance to a predetermined media volume while still allowing full access to the interior of the coil for purposes of loading it with media. The invention further relates to a control circuit for correcting phase error of electrical currents in the coils toward a predetermined optimum phase, for example, a phase of 90.degree., for the case of orthogonal coils such that the vector sum of the magnetic fields uniformly rotates the resultant field in the plane defined by the intersecting axes of the coils. The invention further relates to automatic loading and optimal positioning of the variety of media form factors commonly encountered in the art.
2. Description of the Prior Art
Various degaussing systems for erasing magnetic media are known in the art. For example, although not applied to magnetic media in bulk, U.S. Pat. No. 2,962,560 discloses a system which utilizes decaying AC magnetic field as an optimal degaussing process. The system in the '560 patent applies a damped sinusoidal field along an information-bearing track of a revolving magnetic drum, exposing each point on the track to a field that reverses and decays.
In more recent art, in which individual tracks on magnetic tapes are erased, several factors including the period of a sinusoidal AC field of constant amplitude; the weakening of that field with distance from its source, such as an erase head; and the motion of tape with respect to that source; cooperate to cause the decaying AC effect, which generally serves to reduce tape noise by randomizing the state of the magnetic domains. Disk drives generally re-use tracks by overwriting them; however, application of track-width fields ranging from DC to random bit patterns are also known to have been used for information erasure.
Bulk degaussers are used in general in the erasure of magnetic tape when tape recorders lack such function, in which case a decaying AC field is generally used in order to erase the tape so it can be reused. In the case of a disk drive, bulk degaussing can destroy and thereby protect information, for example, when the write mechanism fails but the read mechanism remains operational. The prior art also extends bulk degaussing to protection of information on tapes. Bulk degaussers erase media faster and more thoroughly than tape recorders and disk drives and relieve the operating stress required for recorders and drives to erase media.
Bulk degaussers generally apply large magnetic fields to media in the form of tape wound on reels in tight spirals approaching concentric cylinders or concentric circular tracks on flat disks stacked in a disk drive. The effectiveness of the intended information erasure depends both on the strength of the magnetic field and its direction or range of directions relative to the information-bearing tracks on the tape and disks. The circular direction of tracks on disks and tape reels is the single most effective direction, and degausser configurations suited to apply that direction to either form can generally apply it to the other form about as well.
The strength of the degaussing field must exceed the variable switching strength of the magnetic domains in the media. Increasing the strength in a single magnetic direction beyond a few multiples of the average switching strength or the coercivity rating of the media fails to improve the erasure effect for some common media formulations. Conversely, the application of a sequence of magnetic directions generally greater than the coercivity erases the media better than a very strong field applied in a single direction.
Given the synergistic effect of radial and axial field directions being about equal to each other, and the fact that nearly cylindrical tape surfaces and circular disk surfaces reverse the orientation of tracks relative to those radial and axial directions, coil systems that rotate magnetic fields in the plane perpendicular to the axes of reel and disk forms generally erase both forms well, and coil systems that add axial directions to the sequence of magnetic fields erase even better.
Many of the prior art bulk degaussing systems employ magnets formed from inductive coil systems that are incapable of generating magnetic field volumes the size of the largest units of magnetic media to be degaussed. Such systems require a relatively long energized time period in order to mechanically transport media through the magnetic field of a coil system and to rotate the field relative to the media. Such art generally draws relatively continuous power from a source, such as an AC supply circuit, to make up for losses, for example, due to the inevitable resistance of the inductive coils. The power supplies for such systems, typically 120 Volts and 15 Amperes, places practical limits on degaussing coil systems, and the resulting strength and volume of the degaussing field that they generate.
Degaussers generating magnetic fields over long periods of time often necessitate energy storage components other than the inductive coils, such as capacitors, to improve the power factor of the inductive coils and to store large quantities of resonant energy in order to increase degaussing strength. Such prior art degaussers are often configured with coils wound on iron cores. These degaussers also generally incorporate switching circuits for limiting operation to periods when media is present or for converting the supplied AC power into a DC current.
A fundamental aspect common to such prior art systems is that the inductive coils always operate in association with their supplemental energy storage and dissipating components with a response forced by their power supplies and the associated switching components. Such responses generally go to the steady state response in a period much shorter than the operating period of the magnet.
Many coil and core geometries have been devised to concentrate, direct and shape small steady state magnetic fields. Likewise, arrays of magnets (coils with or without cores) and many combinations of linear and rotational motion, either of the magnets or of the media or both, have been applied to rotate or otherwise vary the direction of the magnetic field beyond 180.degree. reversals while distributing it over the entire bulk of the magnetic media. Examples of such systems are disclosed in U.S. Pat. Nos. 2,481,392, 3,023,280, 3,938,011, 3,588,623, 4,346,426, 4,467,389, 4,639,821, 4,730,230, 4,751,608, 4,897,759, and 5,204,801.
When the magnetic field is smaller than the media volume, such as in the steady state prior art discussed above, mechanical failures can compromise the degaussing process. Commercial bulk degaussers, such as Model 905-, 930-, 940-, and 943-series, manufactured by Data Security, Inc., disclosed in U.S. Pat. Nos. 4,751,608 and 5,721,665, detect exceptions to such operation. These systems automatically compensate in certain cases where interactions between a load of media and the magnetic field causes predetermined motions to deviate. Even so, a human operator may misinterpret motion variations as an undetected failure, or may ignore warning indicators precipitated by an anomaly for which compensation has not been provided.
U.S. Pat. No. 3,879,754 discloses a system with multiple coils wound orthogonal to each other around cores and operating in forced response to power supplies, which may include AC excitation of one set of coils simultaneous with DC excitation of the orthogonal set of coils. Like the other prior art discussed above, the '754 patent illustrates a system with a small magnetic field volume relative to a much larger moving bulk of magnetic media. The coils of that invention, at least superficially, resemble orthogonal coils that can rotate magnetic fields electronically.
U.S. Pat. No. 4,423,460 discloses a system which rotates a magnetic field electronically using coils wound on orthogonal cores and driven toward a steady state forced response in cooperation with passive phase shifting. Complementary phase shifting of plus and minus 45.degree. is used to improve the power factor from near zero to 0.71, which may serve to reduce line current while increasing field strength. Even so, its media-sized volume and steady state operation limit the field strength that can be generated for a particular supply circuit rating. The system also discloses motion of the media but does not disclose detection nor measurement to place media within or convey it past the rotating field.
U.S. Pat. No. 2,962,560 discloses a system with a switched LC circuit which includes a capacitor initially charged to a voltage for the purpose of storing energy. Switch closure transfers energy into an inductor. Unlike the resonant energy storage function of capacitors in degaussers operating coils to the steady state, a supply circuit of low power delivers energy to charge the capacitor to predefined voltage and energy storage levels over a long period during which the coil is switched off, offering transient degaussing strength and field volume independent of the supply circuit rating.
As long as some inevitable electrical resistance remains well under a value critical to the values of capacitance and inductance, the natural transient response of such under-damped second order series LCR circuitry is an exponentially decaying sinusoid, where the values of inductance and resistance alone dictate the rate of decay, and the values of capacitance and inductance approximately dictate the frequency of the sinusoidal oscillations; the influence of the resistance on that frequency being limited by its value well under the critical damping value.
U.S. Pat. No. 4,551,782 discloses switched LC circuits capable of exponential AC decay, describing the resulting transient operation as "magnetic fields in alternating directions at successively decreasing amplitudes." That system employs one coil to generate a field the size of the media in the direction parallel to the axis of the media. A second coil situated with its axis orthogonal to the first coil is not symmetrically disposed about it or situated to generate a field the size of the media. Therefore, this system does not electrically rotate the field over the direction of the tracks, but rather it rotates the media through the second coil at a rate much faster than the oscillatory period of the sinusoidal field in order to achieve the desired decaying exposure.
The transient natural response of an LC circuit from a state of initial stored energy can be termed a "ringing signal generator" as in U.S. Pat. No. 5,270,899. That fundamental response offers the most direct means for synthesis of the desired decaying AC field. Even if a non-exponential decay rate or non-sinusoidal oscillation were desired, means such as digital synthesis and linear amplification, are not generally suited to the current and voltage levels or the component impedances of practical bulk degaussers. That system applies indexed motion alternating with generation of that decaying AC field smaller than a unit of media to expose all of the media. It measures the size of the media to calculate the number of degaussing positions desired or indexes motion in predetermined increments or both.
While the small transient field of such prior art does limit the energy storage requirement, problems can arise in its practice. First, the calculation of indexed motion increments is not clear-cut in all cases. Second, the application of magnetic fields can exert mechanical forces with the potential to physically move the media. Eddy currents, caused by oscillatory fields, generally act to center or levitate non-ferromagnetic conductors like aluminum housings and thus reduce friction between media and conveyance. Such levitation can complement attraction or repulsion of soft magnetic materials or permanent magnets used in head and spindle motors of disk drives and in a variety of other uses. Thus, if the art is applied to erasing a variety of media formats, a variable potential for positioning errors arises due to unintended media motions.
Addressing such unintended motion errors with mechanical restraint increases the system complexity, especially in the case of processing a large variety of media. Straightforward restraints like straps add to the thickness of media and thus to the volume required for the magnetic field, adversely impacting on the energy storage requirement of such systems.
Some known degaussers, such as manufactured by Sanix Corporation, are known to utilize transient field degaussers with coils that encompass media completely and so do not measure media size to calculate linear erasing positions. They do index rotation to change the direction of the field between successive transient fields.
Users of bulk degaussers view media-sized magnetic fields as a means to minimize failure-induced loss of efficacy. Degaussing systems that electrically rotates such fields do so with multiple coils. In some systems, even if one coil fails, the other coil still exposes the media volume, and even though it generates a field of sub-optimal direction with respect to much of the media volume, it can still erase those regions significantly.
U.S. Pat. No. 3,143,689 discloses a system which completely encompasses the degaussing field volume within a set of closely conforming coils, wound orthogonal to each other. This system requires a drawer for the inner coil in order to load media into that volume. Separable electrical connectors allow the drawer to operate and its coil to connect to a capacitor and switching circuitry. Flexible lead wires to the drawer may be impractical due to necessarily large wire size. Rather than relying on the natural transient response, this system connects capacitors directly to its coils and switches energy from another capacitor into the LC circuits with vacuum tubes driven by an oscillator, thereby providing the capability to force a resonant response in the circuits. It further offers means to vary the biases on the grid voltages relative to each other, which should change the direction of the plus and minus 180.degree. oscillatory field over 90.degree.. Its operation should yield the adequate rotation and transient decay effects if that bias is adjusted between its limits more slowly than the oscillations but more rapidly than circuit losses drain energy from the third capacitor.
U.S. Pat. No. 4,617,603 illustrates a typical solution for the avoidance of separable connectors in order to access a media-sized volume. The windings are disposed over only part of the surface defining the degaussing volume, leaving openings for access to the volume. It has been reported that the rotating field generated by such a system, as embodied in the Rimage Corporation (formerly IXI Corp.) Model 5661C degausser, over a volume sized for 15 inch diameter media and employing such loosely conforming windings suffers significant loss of field strength 5 inches along the axis of either coil in the direction of each axis. This system also introduces microprocessor control coupled with semiconductor switching to force a response in its two LC degaussing circuits from energy stored in another capacitor. It also employs active feedback from a magnetic sensor, although the analogous voltage or current signals might be processed simply to achieve the claimed predetermined phase and amplitude relationships that rotate its field and can allow that field to decay.
The Rimage Corp. Model UDGS-FLOOR degausser exemplifies a variation on electrically rotated transient field bulk degaussing. It features an additional coil set mutually orthogonal to both of the main coils, but wired in series with the LC circuits and switching circuit to a large capacitive energy reservoir. The field direction generated by the additional series connected coils, being orthogonal to the rotating field generated by the main coils, offers the potential of synergistic effects on erasure performance resulting from a wider range of magnetic field directions. The series connection of those coils also allows them to serve as charging reactors for the main LC circuits. Microprocessor monitoring of circuit voltages via A/D conversion in this model can time switching to force the desired phase relationship between the coil currents.